The map shows the mean annual concentrations of ammonium (NH4+), expressed as milligrams of NO2-N per litre of water, observed in groundwater monitoring stations and reported by EEA member countries via WISE. The purpose of the map is to provide an overview of the mean annual value of ammonium in groundwater across Europe and to enable the user to compare values per country or per groundwater body, depending on the scale of visualisation. Historical data since 2000 can be displayed using the time slider, and different horizons can be selected using the layers menu.